A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost power converter is one example of a switched-mode converter that converts the input voltage to an output voltage that is greater than the input voltage. Typically, the boost power converter is employed in off-line applications wherein power factor correction is required and a stable regulated voltage is desired at the output of the power converter.
A non-isolated boost power converter generally includes an energy storage device (e.g., an inductor) coupled between the input voltage and an inverter or power switch. The power switch is then coupled to a rectifier (e.g., a power diode) and an output capacitor. The load is connected in parallel to the capacitor. Again, the output voltage (measured at the load) of the boost power converter is always greater than the input voltage. When the power switch is conducting, the diode is reverse biased thereby isolating the output stage. During this period, the input voltage supplies energy to the inductor. When the power switch is not conducting, the output stage receives the energy stored in the inductor for delivery to the load coupled to the output of the converter.
Analogous to other types of power converters, a boost converter is subject to inefficiencies that impair the overall performance of the power converter. More specifically, the power switch and rectifier are subject to conduction and switching losses that reduce the efficiency of the power converter. Additionally, during the turn-on interval of the power switch, the power diode is also subject to a reverse recovery condition that induces a substantial current spike through the power switch and diode. Furthermore, the power switch e.g., a metal-oxide semiconductor field-effect transistor (MOSFET)! is subject to losses when a charge built-up in the device dissipates during the turn-on transition period of the switch. The losses associated with the power switch and rectifier increase linearly as the switching frequency of the power converter escalates. Therefore, efforts to minimize the losses associated with the power converter and, more specifically, the switching losses associated with the power switch and rectifier will improve the overall efficiency of the power converter.
Efforts to reduce the switching losses associated with the switching devices of the power converter have been the subject of many references. For instance, Novel Zero-Voltage-Transition PWM Converters, by Hua, et al., IEEE Power Electronics Specialists Conference, p. 55-61 (1992) (incorporated herein by reference), addresses the issue of switching losses associated with switching regulators. Hua, et al., attempt to reduce dissipative losses associated with a power switch and diode of a switching regulator. In pertinent part, Hua, et al., couple a auxiliary switch and pilot inductor to a tap of the main inductor of the switching regulator. With the auxiliary switch and inductor, Hua, et al., attempt to limit a rate of the change in current across the diode and decrease a voltage across the power switch to reduce the turn-on switching losses associated with the power switch. While addressing the basic problem associated with the turn-on losses of the power switch, Hua, et al., fail to resolve an inherent contradiction that arises in the operation of the switching regulator.
The contradiction in the design develops for the following reasons. By employing a relatively large pilot inductor, the reverse recovery current through the diode decreases due to the fact that the rate of the change in current through the diode is diminished. The negative effect of employing a relatively large pilot inductor, however, is that it takes a longer period of time to discharge a charge across the power switch thereby extending the time period to achieve zero-voltage switching (ZVS) at the turn-on of the power switch. The extended time period to achieve ZVS defines the on-time of the auxiliary switch resulting in additional conduction losses associated therewith. The opposite situation occurs when a relatively small pilot inductor is employed in the switching regulator. Under these circumstances, the charge across the power switch is discharged at a faster rate to accommodate ZVS, but, in turn, a reduction of the reverse recovery current of the diode is limited and additional switching losses occur in the auxiliary switch. Hua, et al. therefore introduce additional dissipative losses (e.g., conduction loss in the auxiliary switch) in an attempt to achieve ZVS and reduce the reverse recovery condition in the power circuit. Hua, et al., is just one example of a failure adequately to resolve dissipative losses (e.g., conduction loss in the auxiliary switch) associated with the switching regulators.
Accordingly, what is needed in the art is a circuit that reduces the dissipative losses associated with the power train of a power converter that overcomes, among other things, the contradiction inherent in designs presently available.